Several different embodiments of such universal plugs for use in hollow walls as well as in walls of solid material are known. In the first mentioned case the plug is put in a through hole in the support and in the latter case in a dead end hole. When the known plugs are arranged in a through hole in a board it is intended that, at the introduction of the cooperating screw in the inner end portion, the plug should be contracted axially and the legs should be bent or broken preferably radially outwards creating a support at the back side of the board. In order to facilitate the bending of the legs the known plugs are as a rule made from a relatively soft plastic material. This means that the part of the plug on the back side of the board will rotate more or less during the introduction of the screw depending on the design of the plug and the material of the same and the legs will position themselves over each other. Such plugs of soft material mounted in wall boards of plaster have, however, shown to give less withdrawal resistance than plugs made from hard plastic material which plugs do not tend to rotate as much and bear better on the hole edge. Plugs of a harder material also show a considerable increase of moment indicating when the screw is sufficiently tightened which eliminates the risk of damaging the plug or the edge of the hole.
On plugs of hard plastic material the legs will, however, not be easily bent or broken radially outwards. In order to facilitate the breaking outwards, the legs have been provided with bending notches in combination with a slight deviation of the legs. The deviation is achieved by means of the fact that the common sectional form of the legs, the number of which due to manufacturing reasons is limited to two, at the middle portion of the plug, forms an ellipse which in the direction of the end portions transforms into a circle. Such a plug comprising two legs with a middle portion having an elliptical section is known from the European Patent EP-A-0169335.
For use of universal plugs in dead end holes it has been advantageous to make the plug from a relatively hard material which does not have cold flow properties which would cause the plug to lose its grip in the wall as time goes on. The axial contraction of the universal plug in hard wall materials, is not sufficient to ensure the grip of the plugs in the wall. The enlarging of the middle portions of the legs in the plug in said European Patent gives an essential improvement of the expansion properties of such a plug. The expansion of the plug takes place, however, only in two opposite directions and the withdrawal resistance does not reach values corresponding to values for known expansion plugs designed for hard wall materials. These plugs are usually divided in four legs by means of radial slots.